Optics: measuring and testing – By alignment in lateral direction
Reexamination Certificate
2001-08-22
2004-11-02
Smith, Zandra V. (Department: 2877)
Optics: measuring and testing
By alignment in lateral direction
C356S510000, C356S138000
Reexamination Certificate
active
06813022
ABSTRACT:
TECHNICAL FIELD
The invention relates generally to an interferometer system for position measurement and more specifically to an interferometer system and method for improving the accuracy of interferometric measurements.
BACKGROUND ART
A laser interferometer is often used to accurately measure relative displacement between two members in a projection exposure system used to manufacture semiconductor devices. The laser interferometer is used as a measuring apparatus for measuring the coordinates of a wafer stage or mask stage for highly accurate positioning of a semiconductor wafer or reticle relative to stationary projection optics.
A prior art laser interferometer system is shown in
FIGS. 1 and 2
. The interferometer system typically measures a change in position in measurement mirrors
2
X and
2
Y attached to a movable stage S relative to stationary reference mirrors
1
X and
1
Y. One or more laser sources (not shown) generate(s) a beam B of light and direct it toward respective beam splitters BX and BY. The beam splitters BX and BY split the beams B into two beams
3
and
4
. Beam
3
is the portion of each beam B that is reflected by the beam splitter and directed toward respective reference mirrors
1
X and
1
Y. The beams
3
reflect off the reference mirrors
1
X and
1
Y and pass through the beam splitters to give beams C. Beam
4
is the portion of each beam B that passes through the beam splitters and is directed toward respective measurement mirrors
2
X and
2
Y, and is then reflected by the measurement mirrors back to the respective beam splitters. The reflected beams
4
are reflected by the respective beam splitters where they are combined with reflected beams
3
into the combined beams C.
The combined beams C are then directed into respective sensors SX and SY, where they are analyzed to compare the distances represented by beams
3
and
4
. If the measurement mirror
2
X moves relative to the reference mirror
1
X, the intensity of the combined beam periodically increases and decreases as the reflected light from the two paths alternately interferes constructively and destructively. This constructive and destructive interference is caused by the two beams moving in and out of phase. Each half wavelength of movement of the measurement mirror results in a total optical path change of one wavelength and thus, one complete cycle of intensity change. The number of cycle changes indicates the number of wavelengths that the measurement mirror has moved. Therefore, by counting the number of times the intensity of the light cycles between darkest and lightest, the change in position of the measurement mirror can be estimated as an integral number of wavelengths.
Theoretically, if the measurement mirrors
2
X and
2
Y are perfectly planar, and the stage to which they are mounted moves perfectly along the x axis, the
2
Y mirror surface should not change its position along the y axis during x axis movements of the stage, and the beams
3
and
4
should stay perfectly in phase as received by the sensor SY. In reality, among other disturbances that may cause interference between beams
3
and
4
at the sensor SY in this situation, the mirror
2
Y is never perfectly planar (of course the same holds true for mirror
2
X). In practice, these mirrors generally have a polishing error of &lgr;/10 or more which equates, for present semiconductor uses, to up to 60 nm deformations measured from the theoretical plane of the mirror surface.
An example of such a deformation is shown in the
2
X mirror in
FIG. 2
, where the solid line shows the actual deformation of mirror
2
X, and the phantom line
2
XI shows the ideal perfectly flat surface, with the deformation indicated by d. The shift of the reflection point from the ideal plane, caused by the bowing of the mirror by distance d, brings about a measurement error, since the interferometer is no longer measuring from the actual reflection point on the ideal planar surface. This error or deformation can be corrected by a pre-measurement of the reflection surface of the measurement mirror
2
X. A shift of the reflection point caused by deformation of the mirror in the x-z plane is typically averaged because the stage stroke along the z axis is small enough, compared with the beam size, to be less significant than the errors induced by bowing.
U.S. Pat. No. 5,790,253 to Kamiya describes an interferometer system for correcting linearity errors of a moving mirror and stage. Thus, Kamiya can correct for the deformation in the mirror along the long axis of the mirror, which is referred to in the art as correcting “mirror bow”. To correct for mirror bow, Kamiya measures the curvature data of the moving mirror prior to its installation on the stage and stores the data as mapping data. Kamiya takes discrete curving error measurement along the length of the mirror after it has been mounted on the stage. Finally, a main controller creates continuous curvature error data after installation of the mirror on the wafer based on the relationship between the data generated before and after mounting the mirror on the stage. The continuous curvature error data is then used as correction data for more accurately placing the stage.
U.S. Pat. No. 5,363,196 to Cameron also describes an interferometer system for correcting mirror bow of a moving mirror mounted to a stage. Cameron provides two interferometer laser metering devices, either one of which is capable of providing measurement data of the angle of rotation of the stage in the x-y plane, for use by computer controlled servo devices that control the x-y movement of the stage. In a calibration mode, the servo devices may receive data of specific measurements defining the respective values of undesired departures from flatness or straightness of the moving mirror surfaces that are mounted to the stage. The departure data is stored in memory, and may be used by the computer controlled servo devices to compensate for the undesired departures in linearity of the mirrors, during the actual movement and processing phases of the stage. Cameron also discloses that, if desired, an additional interferometer may be provided along each of the x and y axes to measure twist in the moving mirrors. However, because of the long, narrow aspect ration of both of the moving mirrors, Cameron indicates that determination of twist may not be worth pursuing.
Sueyoshi, in Japanese HE19-210648, discloses a method and device for measuring a plane shape at a desired pitch by detecting positional information on the plane along three specified points that are separated by predetermined distances. For example, three x direction interferometers are aligned in the y direction and spaced at predetermined distances along the y direction. A similar arrangement is provide for y direction interferometers. These arrangements are then used to take measurements for a determination of mirror bowing in the x and y reflective mirrors, respectively.
As manufacturers of integrated circuits attempt to increase circuit density and reduce circuit feature size, interferometers are required to provide more precise measurement data. As the circuit density increases, the tolerance for error in alignment of the stage system decreases, so that a shift of the reflection point caused by deformation of a mirror in the x-z or y-z plane also becomes more significant. Additionally, if the stage tilts, a lateral shift of the reflection point occurs which will not be detected by a system for correction of mirror bowing. The result is an error in the position measurement of the stage that results in misalignment of circuit patterns on the wafer (mounted on the stage) relative to one another.
There is, therefore, a need for an interferometer system that measures and corrects for deformation of moving mirrors as well as tilt of the mirrors and tilt of the stage with respect to the z axis.
SUMMARY OF THE INVENTION
The invention provides a measuring system that measures and corrects for deformation and tilt of substantially planar surfaces with respect to a
Finnegan Henderson Farabow Garrett & Dunner LLP
Nikon Corporation
Smith Zandra V.
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